The antennas according to the invention are applied, for example, to equip vehicles and for a frequency band varying from 225 to 400 MHz. They may operate with space diversity, all the antenna elements forming the antenna then operating in one and the same frequency range. The antennas may also consist of several antenna elements operating in mutually different frequency bands. The position of the different antenna elements forming the antenna, relative to one another, depends on the application.
An antenna according to the invention may take the form of a whip, also known as “low profile”, have at least two independent inputs or power supplies, retain an omnidirectional coverage and be predisposed to signal processing of space diversity type.
Hereinafter in the description, the expression “low profile” corresponds to the transversal dimensions of the antenna itself, that is to say its section.
It is known practice to produce a double antenna including a power supply means. For example, FIGS. 1A and 1B (respectively seen in perspective and seen in cross section) represent an antenna system consisting of a first dipole 1 consisting of a top radiating element is and a bottom radiating element 1b in the form of a skirt, a second dipole 2, placed collinearly to the dipole 1 and consisting of a top radiating element 2s in the form of a counter-skirt (overturned skirt) and a bottom element 2b also in the form of a skirt, a first coaxial cable 3 passing through the assembly 2b, 2s, 1b and powering the dipole 1 via the electrical connections of its core 5 with the element 1s and of its sheath 6 with the element 1b, a second coaxial cable 4 powering the dipole 2 via the electrical connections of its core 7 to a quarter-wave trap 9, usually called “stub”, at the level of the point A and of its sheath 8 with the element 2b. The drawbacks with this type of structure notably stem from the use of the stub. In practice, it is known that the effectiveness of the “stub” is governed by the relationship giving its apparent impedanceZstub=Zctg(2πL/λ)
with Zc=60 In (D/d), D being the diameter of the stub, d the apparent diameter of the cables that pass through it, L the length of the stub and X the wavelength.
Since the effectiveness of the stub increases in direct proportion to the apparent impedance Zstub, the result is that, as the bandwidth to be covered becomes wider, the value required for D becomes greater, which runs counter to the search for a low profile for an antenna while retaining a wide antenna bandwidth.
Another double antenna structure is described in the patent FR 2 300 429 and represented in FIG. 2. This antenna system consists of a first dipole 1 consisting of a top radiating element 1s linked to the core 11 of a multi-axial line 12 and a bottom radiating element 1b linked to the sheath 121 of the multi-axial line, a second dipole 2 consisting of a top radiating element 2s linked to the sheath 121 at the point 10 and a bottom radiating element 2b linked to the sheath 122 of the multi-axial line 12. Such a system, while effective, does, however, present the drawback of requiring the implementation, in order to cover a wide frequency band, thick radiating elements, for example, cone sections, disks, etc., which result in an increase in antenna size, which runs counter to one of the objectives sought, namely, to minimize the size of the antenna while retaining a desired bandwidth.